Prof. A. Des Cloiseaux on the three Types of Hum-it e. 287 



of my ' Manual of Mineralogy.' I have placed the obtuse 

 angle of the prism in front, in accordance with the general 

 usage for orthorhombic prisms ; and I have multiplied the old 

 value of the vertical axis by \ to simplify the symbols. 



The plane of the optic axes is parallel to the base ; the acute 

 bisectrix is positive, and is normal to h l (0 1 0). Dispersion 

 hardly appreciable in oil, p < v (?). 2R a . r = 78° 18 / to 79°. 



The laminse parallel to h 1 consist of plates (of greater or less 

 extent) in which the extinction of the light is complete. These 

 are intersected by patches of irregular form in which the ex- 

 tinction is imperfect, and which probably consist of Hmnite 

 belonging to the third type. These probably interfere with 

 the accurate determination of the chemical composition of 

 crystals of the first type. 



Type II. Yellow Humite from Vesuvius, and brown chondro- 

 clite from Sweden (Kafveltorp). 



Oblique ; prism-angle = 50° 2' 40". 



b : a : c : : 419122 : 907930 : 696136. 



Scaccbi's 

 symbols. 



Symbols of 

 Des Cl. 



Miller's 

 symbols. 



Calculated 

 angles. 



Observed 

 angles. 

 Scaccbi. 



r A:C 



P9 X 



01:010 

 001:012 



90 d 

 141 51 



90 d 

 J 141 49 Scacb. 

 1141 48DesCl- 



e 2 pointed out by Scacchi ; refound by vom Rath 

 and myself in chondrodite. 



Art 



pe 1 



001:011 



*122 29 



122 29 



A:e 



pa 1 



001:101 



136 



135 58 



A:e' 



i 



001 :201 



135 56 



135 58 





2 



pa' A 



001:302 



119 56 







po 5 



1:601 



119 50 





ov and a*) vom Rath. 



A:e 2 



i 

 pcC* 



1:201 



109 5 



108 58 



_ A:e 2 



ph l 



001:100 



*108 58 



108 5S 



r- A : m 

 L A:m 2 



pa 2 

 p9> 



1:312 

 001:311 



115 1 

 95 22 



115 

 95 20 



A : n 



py 



001:2 12 



125 5 



125 5 



A : n l 

 o 3 and 

 cl 



P°i 

 y holohed 

 londroditc 



1:211 

 ral in Hu 

 i (Des Cl. 



125 2 



mite (v. ] 



125 (nearly )DesCl. 

 3ath) and in 



